Given two binary strings, return their sum (also a binary string). For example, a = "11" b = "1" Return "100".

Add Binary.cpp

class Solution {
public:
    string addBinary(string a, string b) {
        string c;
        int lenA = a.length() - 1;
        int lenB = b.length() - 1;
        int left = 0;
        while(lenA >= 0 or lenB >= 0 or left != 0) {
            int tmpA = lenA >= 0 ? a[lenA] - '0' : 0;
            int tmpB = lenB >= 0 ? b[lenB] - '0' : 0;
            int sum = tmpA + tmpB + left;
            left = 0;
            if(sum >= 2) {
                ++left;
                sum -= 2;
            }
            c.push_back(sum + '0');
            --lenA;
            --lenB;
        }
        
        for(int i = 0; i < c.length() / 2; ++i) {
            swap(c[i], c[c.length() - i - 1]);
        }
        
        return c;
    }
};